Dump trucks have a dump body adapted to receive and carry a load to a designated dump site where the load is dumped by tilting the dump body. In many instances, however, part or all of the load is trapped in the dump body because of the incompatibility of the physical characteristics of the load and the design of the dump body. In order to dislodge the load, the load has to be dug or scraped out of the interior of the dump body.
Load ejectors have also been utilized to dislodge the load. Typically, load ejectors are attached to the top of the front slope plate of the dump body and lie loosely on the bottom of the dump body so that when the dump body is raised, the load ejectors hang from the head of the dump body causing the load to be dislodged from the bottom of the dump body and ejected. These load ejectors will typically hang substantially vertically to the ground so that the bottom portion of the ejector extends substantially to the end of the dump body. This type of load ejector, however, is ineffective in long or shallow dump bodies because, when the dump body is tilted, the bottom portion of the load ejector sags along the bottom of the dump body, unable to dislodge the load.
In order to dislodge the load in long or shallow dump bodies, the ejectors can be attached to a rotating head frame, pivotally mounted at a pivot point on the dump body, the rotating head frame then moves the end of the ejector to the rear of the dump body in order to prevent the bottom portion of the ejector from sagging along the bottom of the dump body. Although the rotating head frame can be actively rotated using a hydraulic system consisting of a hydraulic cylinder, pump and valves, the system would be expensive and could be difficult to maintain in the construction environment so that passive means have been utilized to rotate the head frame between the body loading and dumping positions.
In the typical passive rotating head frame system, gravity is used to rotate the head frame along an arcuate path between the body loading and dumping positions. Specifically, the center of gravity of the head assembly, consisting of the head frame and the load ejector is located so that it is forward of the head frame pivot point when the dump body is in the loading position and is to the rear of the head frame pivot point when the dump body is in the dumping position. When the dump body is raised, gravity forces the head frame to rotate to the rear of the head frame pivot point thereby moving the load ejector to the rear of the dump body to dislodge the load. Similarly, when the dump body is lowered, gravity forces the head frame to pivot forward of the head frame pivot point thereby returning the load ejector ready to accept another load.
Without a hydraulic system, however, there is no means to control the rotation of the head frame which, when it falls, accelerates and gathers momentum before it impacts on the dump body. It will be appreciated that head frames and load ejectors are large, sturdy and relatively heavy so that tremendous forces and stresses can be created when the dump body is impacted. This can obviously create a dangerous situation to the equipment, which can be damaged by a rotary head frame. Attempts to control the rate of rotation of the head frame as it falls have been largely unsatisfactory.